The present invention relates generally to cellular mobile radiocommunications systems, more particularly to managing radio resources in such systems, and even more particularly to managing radio resources in such systems while handing over a call between cells (referred to as xe2x80x9cinter-cell call handoverxe2x80x9d).
With reference to FIG. 1, the general architecture is recalled of a cellular mobile radiocommunications system such as, in particular, the Global System for Mobile communications (GSM). Such a system essentially comprises:
a plurality base stations or xe2x80x9cBase Transceiver Stationsxe2x80x9d (xe2x80x9cBTSsxe2x80x9d), such as those referenced BTS1, BTS2, BTS3, and BTS4; in the example shown, each of the base stations is assigned to a respective cell, the cells being referenced C1, C2, C3, and C4 (other examples naturally being possible in which the same BTS is assigned to a plurality of cells), and the base stations co-operate with mobile stations, such as MS1, MS2, MS3, MS4, situated in the cells;
a plurality of xe2x80x9cBase Station Controllersxe2x80x9d (xe2x80x9cBSCsxe2x80x9d), such as those referenced BSC1 and BSC2, each of the BSCs controlling a respective subset of cells from the point of view of managing the radio resources; in the example shown, the Base Station Controller BSC1 controls the cells Cl and C2, and the Base Station Controller BSC2 controls the cells C3 and C4; and
a plurality of xe2x80x9cMobile Switching Centersxe2x80x9d (xe2x80x9cMSCsxe2x80x9d), such as that referenced MSC1, each of the MSCs co-operating with external networks, such as, in particular, Public Switched Telephone Networks (PSTNs) or Integrated Services Digital Networks (ISDNs), and controlling a subset of Base Station Controllers from the point of view of managing calls; in the example shown, the Mobile Switching Center MSC1 controls the Base Station Controllers BSC1 and BSC2.
In such a system, a decision to hand over a call is made by the network on the basis of a certain number of parameters, such as, in particular, the results of measurements taken on radio signals for the current server cell and for cells neighboring the current server cell.
The cell to which the call is to be handed over may be referred to as the xe2x80x9cnew server cellxe2x80x9d, and it is more precisely chosen from a set of cells referred to as xe2x80x9ccandidate cellsxe2x80x9d (determined by the network from said neighboring cells on the basis of said measurement results) in the following manner.
Firstly a request is made to hand over the call to the best of the candidate cells. When such handover is possible, the best candidate cell constitutes the new server cell. When such handover is not possible, a request is made to hand over the call to the following candidate cell, in the order of preference of candidate cells, and so on.
A candidate cell to which handover is requested may also be referred to as the xe2x80x9ctarget cellxe2x80x9d.
A target cell to which a requested handover is not possible is also referred to as a xe2x80x9crejected cellxe2x80x9d. A target cell can be rejected in particular because the mobile station does not succeed in setting up a link with the network in the target cell, or else because no radio resource is available in said target cell, because of traffic overload, or because a testing or maintenance operation is in progress in the target cell, this list naturally not being exhaustive.
When the target cell is controlled by the same BSC as the current server cell (in FIG. 1, this situation corresponds, for example, to handover being requested from cell C1 to cell C2, or from cell C3 to cell C4, such handover also being referred to as xe2x80x9cinternal handoverxe2x80x9d or xe2x80x9cintra-BSC handoverxe2x80x9d), the BSC in question is capable, by itself and internally, of determining whether or not handover to the target cell is possible, i.e. whether or not the target cell is a rejected cell, since said BSC manages the radio resources for both of the cells.
When the target cell is controlled by a BSC (referred to as the xe2x80x9ctarget BSCxe2x80x9d) other than the BSC (referred to as the xe2x80x9ccurrent server BSCxe2x80x9d or more simply below as the xe2x80x9cserver BSCxe2x80x9d) that controls the current server cell (also referred to more simply below as the xe2x80x9cserver cellxe2x80x9d), this situation corresponding for example, in FIG. 1, to a request to hand over a call from one or other of the cells C1 and C2 to one or other of the cells C3 and C4 (such handover also being referred to as xe2x80x9cexternal handoverxe2x80x9d, or xe2x80x9cinter-BSC handoverxe2x80x9d), the server BSC is not capable itself and internally of determining whether or not handover to the target cell is possible, i.e. whether or not the target cell is a rejected cell, and the procedure is then as shown in one or other of FIGS. 2 and 3, namely:
the server BSC, referenced BSCs, firstly sends a handover request message (referenced HO REQ in this example for xe2x80x9chandover requiredxe2x80x9d) to the MSC;
the MSC in turn sends a message of the same type (referenced HO REQxe2x80x2 in this example for xe2x80x9chandover requestxe2x80x9d) to the target BSC, referenced BSCc;
after having determined internally whether or not such a handover to the target cell is possible, i.e. whether or not said target cell is a rejected cell, the target BSC sends to the MSC either, when the target cell is not a rejected cell (FIG. 2), a handover request acknowledgment message (referenced HO REQ ACK in this example) itself containing a handover command message, or else, when the target cell is a rejected cell (FIG. 3), a message indicating that it is impossible to perform handover to said target cell (this message being referenced HO FAIL in this example for xe2x80x9chandover failurexe2x80x9d); and
the MSC in turn sends a message of the same type to the server BSC, namely either a handover command message (referenced HO COMM in this example), or else a message indicating that it is impossible to perform handover to said target cell (this message being referenced HO REJ for xe2x80x9chandover request rejectedxe2x80x9d).
As shown in FIGS. 4 and 5, once a handover (or xe2x80x9ctransferxe2x80x9d) T1 from one cell Ci to another cell Cj has actually been performed, it is possible that a request might be made for a handover T2 to a cell Ck that was rejected previously during the handover T1. If the cell Ck was rejected previously, then a handover to it was requested previously, i.e. it was probably one of the best candidate cells, and it is thus normal, in particular if the request for handover T2 is made soon after handover T1, that it reappears as one of the best candidate cells for handover T2, and thus that handover to it is requested once again.
As shown in FIG. 4, when both of the cells Ci and Cj are controlled by the same BSC, referenced BSCij, the BSC is aware of that fact that the cell Ck was rejected during handover T1, and is thus able to decide that, temporarily, a request for such a handover T2 to cell Ck must not be made.
As shown in FIG. 5, when the two cells Ci and Cj are not controlled by the same BSC, the current server BSC, referenced BSCj, is not aware of the fact that cell Ck was rejected during handover T1 (because only the initial server BSC, referenced BSCi, is aware of that fact), and it is thus not able to decide that, temporarily, such a request for handover T2 to cell Ck must not be made.
Thus:
if the cells Cj and Ck are not controlled by the same BSC, then the current server BSC (BSCj) uses the procedure recalled above with reference to FIGS. 2 and 3 to send a handover request to the target BSC (not shown) controlling the cell Ck, and it then might receive from the target BSC a message indicating that it is impossible to perform handover to the cell Ck. An unnecessary handover request will then have been made, thereby lengthening handover time unnecessarily, and giving rise to unnecessary consumption of transmission resources for the purpose of interchanging the corresponding signalling messages, and thus degrading the quality of service; and
if the cells Cj and Ck are controlled by the same BSC, the current server BSC (BSCj) might unnecessarily and internally have to determine whether a handover to the cell Ck is possible, thereby lengthening call handover time unnecessarily, and thus also degrading the quality of service.
A particular object of the present invention is to avoid those drawbacks, i.e. to optimize operation of the network in such cases, and more generally to improve co-operation between entities of the network during inter-cell call handover.
The present invention provides a method of improving co-operation between entities in a cellular mobile radio-communications network, during inter-cell call handover, during which a xe2x80x9cnew server cellxe2x80x9d is chosen from a set of xe2x80x9ccandidate cellsxe2x80x9d as being a candidate cell to which a requested handover is possible, candidate cells to which a requested handover is not possible being referred to as xe2x80x9crejected cellsxe2x80x9d, wherein, during xe2x80x9cexternalxe2x80x9d call handover, namely when handing over a call from a xe2x80x9ccurrent server cellxe2x80x9d controlled by a xe2x80x9ccurrent server entityxe2x80x9d to a xe2x80x9cnew server cellxe2x80x9d controlled by a xe2x80x9cnew server entityxe2x80x9d which is distinct from said current server entity, said new server entity is informed of any previously rejected cells.
Thus, since the new server entity is informed of the previously-rejected cells, it does not attempt unnecessarily to make requests for handover to previously-rejected cells, thereby avoiding the above-mentioned drawbacks.
According to another characteristic, said information as to any previously-rejected cells further includes an indication as to the reason for which the cells were rejected, or as to a lapse of time after which a handover to said previously-rejected cells can, if necessary, be requested again.
Thus, the new server cell is then able to determine the time at which a request can, if necessary, be made again for handover to the previously-rejected cells, thereby making it possible to optimize network operation further.
The present invention also provides a cell control entity for a cellular mobile radiocommunications network, in particular a BSC for a network such as a network of the GSM type, for implementing such a method.